Weighing a quasar’s galaxy with precision

Combining gravitational lensing and quasars.


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While the mass of the black hole is accessible from single-epoch spectra, measuring the mass of its host galaxy is challenging as the active nucleus largely overshines its host. A new study reported a technique to probe quasars.

Researchers from EPFL have discovered a way to use the powerful gravitational lensing phenomenon to accurately measure the mass of a galaxy harboring a quasar and their evolution in cosmic time. This method is around three times more exact than any previous method. Determining the mass of quasar host galaxies offers new insights into the evolution of galaxies in the early universe for building scenarios of galaxy formation and black hole development.

Unlike conventional techniques, which lack precision and are susceptible to biases, the gravitational lensing technique gives a direction for obtaining robust mass estimates in the distant Universe.

Martin Millon, lead author of the study and currently at Stanford University on an SNF grant, said, “The masses of host galaxies have been measured in the past, but thanks to gravitational lensing, this is the first time that the measurement is so precise in the distant Universe.”

“We can determine the mass of the lensing object thanks to gravitational lensing. According to Einstein’s theory of gravitation, we know how enormous objects in the foreground of the night sky, known as the gravitational lens, may bend light emanating from background objects. Strange rings of light appear, resulting in gravitational lens distortions of the light coming from the background object.”

Over ten years ago, while riding his bicycle to the Sauverny Observatory, Corbin had the epiphany that quasars and gravitational lensing could be combined to determine the mass of a quasar’s host galaxy. To do this, he had to locate a quasar in a galaxy that serves as a gravitational lens.

Although the Sloan Digital Sky Survey (SDSS) database was an excellent resource for finding gravitational lensing quasar candidates, Courbin needed to view the lensing rings to be certain. He and his colleagues ordered Hubble Space Telescope time in 2010 to observe four possibilities, of which 3 showed lensing. One of the three, SDSS J0919+2720, stood out among the other two due to distinctive gravitational lensing rings.

Courbin said, “The HST image of SDSS J0919+2720 shows two bright objects in the foreground that each act as a gravitational lens, probably two galaxies in the process of merging.”

“The one on the left is a bright quasar within a host galaxy too dim to be observed. The bright object on the right is another galaxy, the main gravitational lens. A faint object on the far left is a companion galaxy. The characteristic rings are deformed light coming from a background galaxy.”

By carefully analyzing the gravitationally lensed rings in SDSS J0919+2720, it is possible to determine the mass of the two bright objects… in principle. Disentangling the masses of the various objects would have been impossible without the recent development of a wavelet-based lens modeling technique by co-author Aymeric Galan, currently at the Technical University of Münich (TUM), also on an SNF grant.

Journal Reference:

  1. Millon, M., Courbin, F., Galan, A. et al. Strong gravitational lensing by AGNs as a probe of the quasar–host relations in the distant Universe. Nature Astronomy (2023). DOI: 10.1038/s41550-023-01982-2


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